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The present invention relates to electrical separators and to a process
for making them.
An electrical separator is a separator used in batteries and other
arrangements in which electrodes have to be separated from each other
while maintaining ion conductivity for example. The separator is
preferably a thin porous insulating material possessing high ion
permeability, good mechanical strength and long-term stability to the
chemicals and solvents used in the system, for example in the electrolyte
of the battery. In batteries, the separator should fully electronically
insulate the cathode from the anode. Moreover, the separator has to be
permanently elastic and to follow movements in the system, for example in
the electrode pack in the course of charging and discharging.
This object is achieved by an electric separator according to the
invention, comprising a sheetlike flexible substrate having a
multiplicity of openings and having a coating on and in said substrate,
the material of said substrate being selected from woven or non-woven
electrically nonconductive fibers of glass or ceramic or a combination
thereof and said coating being a porous electrically insulating ceramic
coating, characterized by a thickness of less than 100 .mu.m.

26. A separator comprising a sheetlike flexible substrate having a
multiplicity of openings and having a coating on and in said substrate,
the material of said substrate being woven electrically nonconductive
fibers of glass and said coating being a porous electrically insulating
ceramic coating, wherein the substrate is a woven glass fiber fabric
comprising woven fibers or filaments which has been produced from 2 to 20
tex yarns and has from 5 to 30 weft threads/cm and from 5 to 30 warp
threads/cm, and the separator has a thickness of less than 100 .mu.m.

27. The separator of claim 26, wherein the separator has a thickness of
less than 50 .mu.m.

28. The separator of claim 26, wherein said fibers or filaments are at
least one glass selected from the group consisting of E-, R- and S-glass.

29. The separator of claim 28, wherein said filaments are coated with
SiO.sub.2, ZrO.sub.2, Al.sub.2O.sub.3 or mixtures thereof.

30. The separator of claim 28, wherein said woven glass fiber fabric was
produced from 5.5 or 11 tex yarns.

31. The separator of claim 26, wherein said coating on and in said
substrate comprises an oxide, nitride or carbide of the metals Al, Zr,
Si, Sn, Ce, Mg, Hf, B and/or Y.

32. The separator of claim 26, wherein the separator has a breaking
strength of 5 N/cm to 500 N/cm.

33. The separator of claim 26, wherein the separator is bendable around a
radius down to 100 mm without damage.

34. A process for producing a separator as claimed in claim 1, the process
comprising providing a sheetlike flexible substrate having a multiplicity
of openings with a coating on and in said substrate, the material of said
substrate being a woven fabric comprising woven glass fibers which has
been produced from threads having a linear density of not more than 20
tex and has from 5 to 30 weft threads/cm and from 5 to 30 warp threads/cm
and said coating being a porous electrically insulating ceramic coating.

35. The coating of claim 34, wherein said coating is provided by applying
to said substrate a suspension comprising at least one inorganic
component comprising a compound of at least one metal, one semimetal or
one mixed metal with at least one element of the 3.sup.rd to 7.sup.th
main group and a sol and heating one or more times to solidify said
suspension comprising at least one inorganic component on or in or else
on and in the support.

37. The process of claim 34, wherein said suspension, which comprises at
least one inorganic component and at least one sol, at least one
semimetal oxide sol or at least one mixed metal oxide sol or a mixture
thereof, is prepared by suspending at least one inorganic component in at
least one of these sols.

38. The process of claim 37, wherein said sols are obtained by hydrolyzing
at least one metal compound, at least one semimetal compound or at least
one mixed metal compound using water, water vapor, ice, alcohol or an
acid or a combination thereof.

39. The process of claim 38, wherein said metal compound hydrolyzed is at
least one metal alkoxide compound or at least one semimetal alkoxide
compound selected from the alkoxide compounds of the elements Zr, Al, Si,
Sn, Ce and Y or at least one metal nitrate, metal carbonate or metal
halide selected from the metal salts of the elements Zr, Al, Si, Sn, Ce
and Y.

41. The process of claim 37, wherein the mass fraction of said suspended
component is 0.1 to 500 times that of the sol used.

42. The process of claim 34, wherein said suspension on and in said
substrate is solidified by heating to 50 to 1000.degree. C.

43. The process of claim 42, wherein said heating is carried out at 50 to
100.degree. C. for 10 min to 5 hours.

44. The process of claim 43, wherein said heating is carried out at 100 to
800.degree. C. for 1 second to 10 minutes.

45. A battery, which comprises the separator as claimed in claim 1.

Description

[0001] An electric separator is a separator used in batteries and other
arrangements in which electrodes have to be separated from each other
while maintaining ion conductivity for example.

[0002] The separator is a thin porous insulating material possessing high
ion permeability, good mechanical strength and long-term stability to the
chemicals and solvents used in the system, for example in the electrolyte
of the battery. In batteries, the separator should fully electronically
insulate the cathode from the anode. Moreover, the separator has to be
permanently elastic and to follow movements in the system, for example in
the electrode pack in the course of charging and discharging.

[0003] The separator is a crucial determinant of the use life of the
arrangement in which it is used, for example battery cells. The
development of a rechargeable battery having a lithium electrode
(negative mass) is desirable. However, commercially available separators
are not suitable for that purpose.

[0004] General information about electric separators and lithium ion
batteries may be found for example at J. O. Besenhard in "Handbook of
Battery Materials"; VCH-Verlag, Weinheim 1999, (chapter 10 pages 553 et
seq.) and at Wakihara and Yamamoto in "Lithium Ion Batteries"
Wiley-VCH-Verlag, Weinheim 1998.

[0005] Separators currently used in lithium ion cells (batteries)
predominantly consist of porous organic polymer films. These are produced
by various companies either by the dry process or by the wet process. The
most important producers are Celgard, Tonen, Ube, Asahi and Mitsubishi.

[0006] The lithium ion cell separators produced by these companies are all
based on the polyolefins polyethylene (PE) or polypropylene (PP). These
are also the separator materials present in all commercially available
lithium batteries. Incidentally, here and hereinafter, the term
"batteries" comprehends secondary and primary lithium battery systems.

[0007] Other possible separator materials are nonwovens composed of glass
or ceramic materials or else ceramic papers. However, these materials
have poor machine processing properties and so are not used at present in
any commercially available battery systems.

[0008] Disadvantages of organic polyolefin separators are their relatively
low thermal stability limit of distinctly below 150.degree. C. and also
their low chemical stability in inorganic battery cells. When used in
lithium batteries, polyolefins are gradually attacked by the lithium. In
systems comprising a polymer electrolyte, a dense oxidation product layer
is formed. It prevents further destruction of the separator in lithium
ion batteries.

[0009] High energy batteries or high performance batteries can simply not
be fabricated using polymer electrolytes, since their conductivity is too
low at the operating temperatures in question. These battery systems
utilize nonaqueous and nonpolymeric electrolytes such as for example
liquid sulfur dioxide. Polymeric separators are not chemically stable in
these electrolytes, being destroyed after some time. These systems
therefore utilize inorganic separators (glass mat, ceramic mat and
ceramic paper) having the familiar disadvantages. These are in particular
that inorganic ceramic or glass mats cannot be machine processed into
wound cells, since they always break at the given pulling tensions.
Ceramic papers are very brittle and cannot be wound or processed into
wound cells for that reason. Utility is therefore restricted to the
production of prismatic cells, where the electrodes/separators are not
wound but stacked. Nor is it necessary in this arrangement for the
materials to have breaking strength.

[0010] There have been initial attempts to use inorganic composite
materials as separators. For instance, DE 198 38 800 encompasses an
electric separator comprising a sheetlike flexible substrate having a
multiplicity of openings and having a coating on said substrate, the
material of said substrate being selected from metals, alloys, plastics,
glass and carbon fiber or a combination thereof and said coating being a
twodimentionally continuous porous, electrically nonconducting ceramic
coating. The separators, which have a support of electrically conductive
material (as reported in the example), however, have been determined to
be unsuitable for lithium ion cells, since the coating cannot be produced
over a large area without flows at the thickness described and
consequently short circuits can occur very easily. A polymeric support,
in contrast, dissolves, since the electrolyte comes into contact with the
substrate.

[0011] It can be stated in summary that there is at present no suitable
separator material for producing wound inorganic high performance or high
energy batteries in an economical manner.

[0012] It is an object of the present invention to provide a flexible,
robust separator for high performance and high energy batteries that has
a low resistance in conjunction with the electrolyte, is bendable and is
machine processible into wound cells. It shall moreover possess
correspondingly good long-term stability in inorganic battery systems to
all battery components (electrolytes, conducting salts, overcharge
products or lithium and the like).

[0013] It was found that, surprisingly, an electric separator comprising a
sheetlike flexible substrate having a multiplicity of openings and having
a coating on and in said substrate, the material of said substrate being
selected from woven or non-woven electrically nonconductive fibers of
glass or ceramic or a combination thereof and said coating being a porous
electrically insulating ceramic coating, and which has a thickness of
less than 100 .mu.m and is bendable, has a sufficiently low resistance in
conjunction with the electrolyte and yet possesses sufficient long-term
stability.

[0014] The present invention accordingly provides an electric separator as
claimed in claim 1, comprising a sheetlike flexible substrate having a
multiplicity of openings and having a coating on and in said substrate,
the material of said substrate being selected from woven or non-woven
electrically nonconductive fibers of glass or ceramic or a combination
thereof and said coating being a porous electrically insulating ceramic
coating, characterized by a thickness of less than 100 .mu.m.

[0015] The present invention likewise provides a process as claimed in
claim 12 for producing a separator as claimed in at least one of claims 1
to 11, which comprises providing a sheetlike flexible substrate having a
multiplicity of openings with/and a coating on or in said substrate, the
material of said substrate being selected from woven or non-woven
electrically nonconductive fibers of glass or ceramic or a combination
thereof and said coating being a porous electrically insulating ceramic
coating.

[0016] In general, the larger the pores, the lower the resistance which
ensues.

[0017] Moreover, the porosity of the separator can be influenced through
the choice of suitable particles, and this similarly leads to modified
properties. A separator parameter which is frequently quoted in this
context is the Gurley number. It is a measure of the gas permeability of
the dry porous separator. As described by O. Besenhard in the Handbook of
Battery Materials, the conductivity of a known system can be inferred
directly from the Gurley number. In generalized terms, a higher gas
permeability (Gurley number) will result in a higher conductivity for the
wetted separator in the battery cell. The Gurley numbers of commercially
available separators range from 10, when the pore diameter is around 0.1
.mu.m, to 30, when the pore diameter is around 0.05 .mu.m (G. Venugopal;
J. of Power Sources 77 (1999) 34-41).

[0018] However, it must always be borne in mind that an extremely large
Gurley number can also be evidence of defects, ie large holes, in the
separator. These defects can lead to an internal short circuit in
operation of a battery. The battery can then very rapidly self-discharge
in a hazardous reaction. In the process, large electric currents occur
that may even cause a contained battery cell to explode in the extreme
case. For this reason, the separator can make a decisive contribution to
the safety, or lack of safety, of a lithium battery. Therefore, the
separator is a decisive structural component of a battery and deserving
of a great deal of attention.

[0019] Polymeric separators do provide the safety performance required at
present by impeding any ionic transport between the electrodes beyond a
shutdown temperature, which is about 120.degree. C. This is because, at
this temperature, the pore structure of the separator collapses and all
the pores close up. As a result of ionic transport being stopped, the
hazardous reaction which can lead to an explosion ceases. However, if the
cell is further heated owing to external circumstances, the breakdown
temperature is exceeded at about 150 to 180.degree. C. At this
temperature, the separator starts to melt and contract. The two
electrodes then come into direct contact at many locations in the battery
cell and so there is an internal short circuit over a large area. This
leads to an uncontrolled reaction which ends with the cell exploding, or
the resultant pressure is released by an overpressure valve (a bursting
disk), frequently with signs of fire.

[0020] The inorganic separator of the invention does not give rise to
these two effects. This has the advantage that battery cells can be
operated at higher temperatures without their shutting down or posing a
safety hazard as a result. The inorganic separator cannot melt and so can
never give rise to a large-area internal short circuit in a cell either.

[0021] The same applies to an internal short circuit due to an accident.
If, for example, a nail were to puncture a battery, the following would
happen according to the type of separator:

[0022] A polymeric separator would melt at the site of puncture (a short
circuit current flows through the nail and causes it to heat up) and
contract. As a result, the short circuit location would increase in size
and the reaction would run away. The inorganic separator of the invention
would not melt. So the reaction in the interior of the cell following
such an accident would proceed very much more moderately. This cell is
thus distinctly safer than one with a polymeric separator. This is an
important factor especially in the transportation sector, such as the
automotive sector.

[0023] Higher permitted operating temperatures are not necessary nor
desirable for lithium batteries in applications such as notebooks,
notepads or cellphones. The situation is different in the transportation
sector, however.

[0024] The automotive industry is trying to contribute to environmental
and resource conservation by increasingly reducing motor fuel
consumption. A relatively efficient method in this connection is the
intermediate storage of braking energy. This can be done by returning the
braking energy to the battery in the case of hybrid vehicles, which have
a battery-operated electrical drive as well as a fuel cell and a
gasoline, diesel or any other engine, or in the case of electric
vehicles. Very large amounts of energy are introduced within very short
times, which can lead to a short sharp increase in battery temperature.
In the case of the use of polymeric separators, the return feed can only
take place when the temperature of every cell is very closely monitored
and the feeding is interrupted when a relatively low temperature limit is
exceeded. When an inorganic separator is used, there is no need for an
interruption, since the high temperature does not have adverse effects on
the separator. As a result, the entire braking energy can be fed into the
battery, and not just a part as in the case of polymeric separators. The
situation is similar with the rapid chargeability of batteries containing
an inorganic separator, as will be appreciated. They are rapidly
chargeable. This is a distinct advantage in the case of use in electric
vehicles, since these no longer have to be charged over 12 h or even
longer time periods; instead charging is feasible within distinctly
shorter periods.

[0025] Owing to the thickness of less than 100 .mu.m for the electric
separators of the present invention, the electric resistance on using the
separator is distinctly lower than in the case of separators known to
date. Despite the low thickness, the separator according to the invention
possesses sufficient strength (breaking strength) of more than 10 N,
preferably of more than 25 N and most preferably of more than 50 N and is
flexible. The breaking strength of commercially available polymeric
separators is about 50 N in the machine direction and 5 N in the cross
direction (G. Venugopal; J. of Power Sources 77 (1999) 34-41).

[0026] The separator according to the invention will now be described
without the invention being limited thereto.

[0027] The electric separators of the invention comprise a sheetlike
flexible substrate having a multiplicity of openings and having a coating
on and in said substrate, the material of said substrate being selected
from woven or non-woven electrically nonconductive fibers of glass or
ceramic or a combination thereof and said coating being a porous
electrically insulating ceramic coating. The separator has a thickness of
less than 100 .mu.m, preferably of less than 75 .mu.m and most preferably
of less than 50 .mu.m.

[0028] The low thickness provides a particularly low electric resistance
for the separator in use with an electrolyte. The separator itself, of
course, has an infinitely large resistance, since it itself must have
insulating properties.

[0029] In order that a separator having insulating properties may be
obtained, the material for the substrate preferably comprises
electrically nonconductive fibers selected from glass, alumina,
SiO.sub.2, SiC, Si.sub.3N.sub.4, BN, B.sub.4N, AIN, sialons or ZrO.sub.2.

[0030] The material of the substrate can be a woven, nonwoven or felt of
electrically nonconductive fibers. In order that a uniform resistance may
be obtained in use with an electrolyte, the material of the substrate is
preferably a glass fiber fabric uniformly woven from glass fibers. This
provides uniform resistance in use with an electrolyte, based on the
surface area of the separator. When a non-woven glass fiber material is
used, it may happen that the separator surface has regions having a
larger resistance in use with the electrolyte and other regions having a
smaller resistance in use with the electrolyte. Such a nonuniform
resistance distribution across the surface would lead to an unnecessary
power loss of the battery.

[0031] In principle, all glass materials available as fibers are usable
for the substrate, for example E-, A-, E-CR-, C-, D-, R-, S- and M-glass.

[0032] Preference is given to using fibers of E-, R- or S-glass. The
preferred glass varieties contain a low level of BaO, Na.sub.2O or
K.sub.2O. Preferably the preferred glass varieties contain less than 5%
by weight and most preferably less than 1% by weight of BaO, less than 5%
by weight and most preferably less than 1% by weight of Na.sub.2O and
less than 5% by weight and most preferably less than 1% by weight of
K.sub.2O. It can be advantageous for the fibers to be made of glass
varieties containing none of the compounds BaO, Na.sub.2O or K.sub.2O,
for example E-glass, since such glass varieties are more stable to the
chemicals used as electrolytes.

[0033] In a particularly preferred embodiment of the separator according
to the invention, the substrate comprises fibers or filaments,
particularly preferably glass fibers or glass filaments of E- or S-glass
coated with SiO.sub.2, Al.sub.2O.sub.3, TiO.sub.2 or ZrO.sub.2 or with
mixtures thereof.

[0034] When the separator according to the invention comprises a substrate
composed of a glass fiber textile, for example a woven, felt or nonwoven
glass fiber fabric, these have preferably been produced from threads
having a linear density of not more than 20 tex (mg/m), preferably from
threads having a linear density of not more than 10 tex and most
preferably from threads having a linear density of not more than 5.5 tex.

[0035] Most preferably, the substrate comprises a glass fiber fabric woven
from threads having a linear density of 5.5 or 11 tex. The individual
filaments or fibers of such threads have a diameter of 5 to 7 .mu.m for
example. The woven glass fiber fabric which is preferably used as a
support has from 5 to 30 weft threads/cm and from 5 to 30 warp
threads/cm, preferably from 10 to 30 weft threads/cm and from 10 to 30
warp threads/cm and most preferably from 15 to 25 weft threads/cm and
from 15 to 25 warp threads/cm. The use of such glass wovens ensures that
the separator is sufficiently strong while possessing sufficient
substrate porosity.

[0036] The separator of the invention comprises a porous electrically
insulating coating. The porosity of the separator is preferably in the
range from 10% to 50%, more preferably in the range from 15% to 50% and
particularly preferably in the range from 25% to 40%. The coating on and
in the substrate comprises an oxide of the metals Al, Zr, Si, Sn, Ce
and/or Y, preferably an oxide of the metals Al, Si and/or Zr.

[0037] The separators according to the invention may have a breaking
strength of greater than 5 and preferably of 20 to 500 N/cm and most
preferably of greater than 50 N/cm. The separators according to the
invention are preferably bendable around a radius of down to 500 mm, more
preferably down to 100 mm, more preferably down to 25 mm and most
preferably down to 5 mm without damage. The high breaking strength and
the good bendability of the separator according to the invention has the
advantage that changes in electrode geometry which occur in the course of
the charging and discharging of a battery can be followed by the
separator without the separator being damaged.

[0038] The separator according to the invention is preferably obtainable
by a process for producing a separator that comprises providing a
sheetlike flexible substrate having a multiplicity of openings with a
coating on and in said substrate, the material of said substrate being
selected from woven or non-woven electrically nonconductive fibers of
glass or ceramic or a combination thereof and said coating being a porous
electrically insulating ceramic coating.

[0039] The coating is preferably applied to the substrate by applying to
said substrate a suspension comprising at least one inorganic component
comprising a compound of at least one metal, one semimetal or one mixed
metal with at least one element of the 3rd to 7th main group and a sol
and heating one or more times to solidify said suspension comprising at
least one inorganic component on or in or else on and in the support. The
process itself is known from WO 99/15262, but not all of the parameters
and starting materials, especially electrically nonconductive starting
materials, can be used for producing the separator of the invention. The
choice of starting materials also dictates certain process parameters
which first had to be found for the combinations of materials chosen.

[0042] The material of the substrate can be a woven, nonwoven or felt of
electrically nonconductive fibers of the abovementioned materials. In
order that a uniform resistance may be obtained in use with an
electrolyte, the material of the substrate is preferably a glass fiber
fabric made from woven glass fibers.

[0043] In principle, all glass materials available as fibers are usable
for the substrate, for example E-, A-, E-CR-, C-, D-, R-, S- and M-glass.
Preference is given to using fibers of E-, R- or S-glass. The preferred
glass varieties contain a low level of BaO, Na.sub.2O or K.sub.2O.
Preferably the preferred glass varieties contain less than 5% by weight
and most preferably less than 1% by weight of BaO, less than 5% by weight
and most preferably less than 1% by weight of Na.sub.2O and less than 5%
by weight and most preferably less than 1% by weight of K.sub.2O. It can
be advantageous for the fibers to be made of glass varieties containing
none of the compounds BaO, Na.sub.2O or K.sub.2O, for example E-glass,
since such glass varieties are more stable to the chemicals used as
electrolytes.

[0044] In a particularly preferred embodiment of the separator according
to the invention, the fibers, particularly preferably the E- or S-glass
fibers, of the substrate are coated with SiO.sub.2, Al.sub.2O.sub.3,
TiO.sub.2 or ZrO.sub.2 or mixtures thereof.

[0045] Such a coating can be applied for example by applying tetraethyl
orthosilicate (TEOS) to the fibers, individually or in the form of woven
fabric, felt or nonwoven fabric, drying the TEOS and then baking the TEOS
at from 400 to 500.degree. C., preferably at from 430 to 470.degree. C.
and most preferably at from 445 to 455.degree. C. The baking leaves
silicon dioxide behind as a residue on the fiber surface. It has been
determined that fibers treated in this way are substantially more useful
as a substrate material than untreated fibers, since the subsequent
coating has substantially better adhesion on the treated fibers and hence
the long-term stability but also the bendability of the separator is
distinctly improved. Coatings with ZrO.sub.2 or Al.sub.2O.sub.3 can be
applied to the fiber surface in the same way, in which case zirconium
acetylacetonate in ethanol is an example of a useful starting material
for producing a ZrO.sub.2 coating and aluminum ethoxide in ethanol is an
example of a useful starting material for producing an Al.sub.2O.sub.3
coating.

[0046] When the substrate is composed of a glass fiber textile, for
example a woven glass fiber fabric, such fabrics have preferably been
produced from threads having a linear density of not more than 20 tex
(mg/m), preferably from threads having a linear density of not more than
10 tex and most preferably from threads having a linear density of not
more than 5.5 tex. Preferred fiber when using a felt or nonwoven glass
fiber fabric has a thickness of 5 to 10 .mu.m, most preferably 5 to 7
.mu.m. Most preferably, the substrate comprises a glass fiber fabric
woven from threads having a linear density of 5.5 or 11 tex.

[0047] The individual filaments of the threads have a diameter of 5 to 7
.mu.m for example. The woven glass fiber fabric which is preferably used
as a substrate has from 5 to 30 weft threads/cm and from 5 to 30 warp
threads/cm, preferably from 10 to 30 weft threads/cm and from 10 to 30
warp threads/cm and most preferably from 15 to 25 weft threads/cm and
from 15 to 25 warp threads/cm. The use of such glass wovens ensures that
the separator is sufficiently strong while possessing sufficient
substrate porosity.

[0048] It has also been determined, when using commercial glass fiber
textiles, especially glass fiber nonwovens, glass fiber felts or glass
fiber wovens, that removal of the size applied to the fibers in the
course of the manufacturing process of the glass fiber textile has a
substantial influence on the strength of the glass fiber textile. The
size is customarily removed by heating the glass fiber textile,
especially a woven glass fiber fabric, to 500.degree. C. and then
thermally treating the textile at >300.degree. C. for several hours to
days. It has been determined that, surprisingly, a glass fiber textile
which has been treated in this way is substantially more brittle than a
glass fiber textile which still has the size. However, the coating
according to the invention is very difficult to apply atop and into a
sized glass fiber textile substrate, since the coating according to the
invention has worse adhesion to the textile because of the size. It was
found that, surprisingly, burning off the size at below 500.degree. C.,
preferably below 450.degree. C., in the course of 2 min and preferably in
the course of 1 min and subsequent treatment with TEOS as described above
is sufficient to ensure a more durable coating of the glass fiber
textile, especially of the woven glass fiber fabric, coupled with
sufficiently good mechanical properties. Removal of the size using
solvents or detergent mixtures in water is likewise possible under
particular conditions.

[0049] The suspension used for preparing the coating comprises at least
one inorganic component and at least one sol, at least one semimetal
oxide sol or at least one mixed metal oxide sol or a mixture thereof, and
is prepared by suspending at least one inorganic component in at least
one of these sols.

[0050] The sols are obtained by hydrolyzing at least one compound,
preferably at least one metal compound, at least one semimetal compound
or at least one mixed metal compound using at least one liquid, solid or
gas. It can be advantageous to use for example water, alcohol or an acid
as a liquid, ice as a solid or water vapor as a gas, or at least a
combination of these liquids, solids or gases. It can similarly be
advantageous for the compound to be hydrolyzed to be introduced into
alcohol or an acid or a combination thereof prior to hydrolysis. The
compound to be hydrolyzed is preferably at least one metal nitrate, metal
chloride, metal carbonate, metal alkoxide compound or at least one
semimetal alkoxide compound, particularly preferably at least one metal
alkoxide compound, one metal nitrate, one metal chloride, one metal
carbonate or at least one semimetal alkoxide compound selected from the
compounds of the elements Zr, Al, Si, Sn, Ce and Y or the lanthanoids and
actinoids, for example zirconium alkoxides, e.g. zirconium isopropoxide,
silicon alkoxides, or a metal nitrate, for example zirconium nitrate.

[0051] It can be advantageous to effect the hydrolysis of the compounds to
be hydrolyzed with at least half the molar ratio of water, water vapor or
ice, based on the hydrolyzable group of the hydrolyzable compound.

[0052] The hydrolyzed compound can be peptized by treatment with at least
one organic or inorganic acid, preferably with a 10-60% organic or
inorganic acid, more preferably with a mineral acid selected from
sulfuric acid, hydrochloric acid, perchloric acid, phosphoric acid and
nitric acid or a mixture thereof.

[0053] Not just sols prepared as described above can be used, but also
commercially available sols, for example zirconium nitrate sol or silica
sol.

[0054] It can be advantageous to suspend in at least one sol at least one
inorganic component having a particle size of 1 to 10 000 nm, preferably
of 1 to 10 nm, 10 to 100 nm, 100 to 1000 nm or 1000 to 10 000 nm, more
preferably 250 to 1750 nm and most preferably 300 to 1250 nm. The use of
inorganic components having a particle size of 300 to 1250 nm provides a
separator possessing particularly useful flexibility and porosity.

[0055] Preference is given to suspending an inorganic component comprising
at least one compound selected from metal compounds, semimetal compounds,
mixed metal compounds and metal mixed compounds with at least one of the
elements of the 3rd to 7th main group or comprising at least a mixture of
these compounds. Particular preference is given to suspending at least
one inorganic component comprising at least one compound of the oxides of
the transition group elements or the elements of the 3rd to 5th main
group, preferably oxides selected from the oxides of the elements Sc, Y,
Zr, Nb, Ce, V, Cr, Mo, W, Mn, Fe, Co, B, Al, In, TI, Si, Ge, Sn, Pb and
Bi, for example Y.sub.2O.sub.3, ZrO.sub.2, Fe.sub.2O.sub.3,
Fe.sub.3O.sub.4, SiO.sub.2, Al.sub.2O.sub.3.

[0056] The mass fraction of the suspended component is preferably 0.1 to
500 times, more preferably 1 to 50 times and most preferably 5 to 25
times that of the sol used.

[0057] It is possible to optimize the cracklessness of the separator
according to the invention through suitable choice of the particle size
for the suspended compounds as a function of the size of the pores, holes
or intermediate spaces in the substrate, but also through the layer
thickness of the separator according to the invention and also through
the ratio of the sol/solvent/metal oxide fractions. The type and size of
the suspended compound used is decisive for the properties of the
separator. The particle size fixes directly the average pore radius and
hence the resistance of the electrolyte-wetted separator. The type of
compound used is decisive for the wettability of the separator by the
electrolyte. The better the wettability of the separator, the better the
separator is suitable for use as a separator. In addition, this separator
can be adapted to the particular electrolyte used by targeted
modification of the surface. It is thus possible to custom tailor the
properties.

[0058] The coating according to the invention is applied to the substrate
by solidifying the suspension in and on the substrate. According to the
invention, said suspension on and in said substrate is solidified by
heating to 50 to 1000.degree. C. In a particular embodiment of the
process according to the invention, the suspension present on and in the
support is solidified by heating to 50 to 100.degree. C., preferably by
heating to 300 to 700.degree. C. and most preferably by heating to 350 to
390.degree. C., 400 to 440.degree. C., 450 to 490.degree. C., 500 to
540.degree. C., 550 to 590.degree. C. or 600 to 650.degree. C. It can be
advantageous to heat at 50 to 100.degree. C. for 10 min to 5 hours or to
heat at 100 to 800.degree. C. for 1 second to 10 minutes. The heating of
the suspension to solidify it is particularly preferably carried out at
300 to 700.degree. C. for 0.5 to 2 min and preferably from 0.75 to 1.5
min. The heating of the suspension to solidify it is most preferably
carried out at 400 to 440.degree. C. for 1.5 to 1 min, at 450 to
490.degree. C. for 1.4 to 0.9 min, at 500 to 540.degree. C. for 1.3 to
0.8 min and at 550 to 590.degree. C. for 1.2 to 0.7 min.

[0059] The composite can be heated according to the invention using heated
air, hot air, infrared radiation or microwave radiation.

[0060] The process according to the invention can be carried out for
example by unrolling the substrate off a roll, passing it at a speed of 1
m/h to 2 m/s, preferably at a speed of 0.5 m/min to 20 m/min and most
preferably at a speed of 1 m/min to 5 m/min through at least one
apparatus which applies the suspension atop and into the support and at
least one further apparatus whereby the suspension is solidified on and
in the support by heating, and rolling the separator thus produced up on
a second roll. This makes it possible to produce the separator according
to the invention in a continuous process.

[0061] Separators according to the invention can be used as separators in
batteries. When the separator is used according to the invention as a
separator in batteries, the separator is customarily placed between the
anode and the cathode in the electrolyte-saturated form.

[0062] The separator according to the invention is suitable for secondary
(rechargeable) lithium batteries. The separator according to the
invention is useful inter alia as a separator in batteries utilizing the
system Li/LiAlCl.sub.4.times.SO.sub.2/LiCoO.sub.2. These batteries have a
lithium cobalt oxide electrode (positive mass) into which lithium ions
can reversibly intercalate and de-intercalate (intercalation electrode).
As the system charges, the lithium ions de-intercalate from the
intercalation electrode and are deposited metallically, generally
dendritically, on a current collector (negative mass). It will be
appreciated that other lithium battery systems are similarly achievable
with the separator according to the invention. Such as, for example,
systems with lithium manganese oxide or lithium nickel oxide as
intercalation electrodes, with carbon materials as negative mass which
reversibly intercalate the lithium, and other electrolytes, such as
LiPF.sub.6 and the like.

[0063] The separator according to the invention is, however, not limited
to such battery systems, but can also be used for example in systems such
as nickel metal hydride, nickel-cadmium or lead accumulators.

[0064] The separator according to the invention is particularly useful in
battery systems having relatively high permitted operating temperatures,
for example lithium batteries, in the automotive sector. As mentioned at
the beginning, the use of separators according to the invention makes it
possible to return braking energy without interruption, since there is no
risk of adverse effects on the separator of the high temperature
occurring in the course of the feeding. As a result, the entire braking
energy can be fed into the battery and not just a part as in the case of
polymeric separators.

[0065] The separators according to the invention are similarly useful in
batteries having a fast charging cycle. By virtue of the high thermal
stability of the separator according to the invention, a battery equipped
with this separator has a distinctly faster charging cycle. This is a
distinct advantage when thus equipped batteries are used in electric
vehicles, since they no longer have to be charged for more than 12 hours
or even longer and instead charging is feasible within distinctly shorter
periods.

[0066] Various chemical and engineering requirements can be met by
adapting the starting materials or by aftertreating the ceramic layer.
For instance, a hydrophilic or hydrophobic coating can be produced by
aftertreatment or by reaction with appropriate chemical groups which are
known to one skilled in the art.

[0067] The nonlimiting examples which follow illustrate the present
invention.

EXAMPLE 1.1

[0068] A glass beaker is charged with 300 g of DM water together with 50 g
of ethanol and 1.2 g of zirconium acetylacetonate. 280 g of
Al.sub.2O.sub.3 (ct 3000 from Alcoa, Ludwigshafen) are then added a
little at a time with stirring. The suspension thus obtained is stirred
for about 24 h. 150 g of zirconium sol (from MEL) are then added and the
whole system is thoroughly stirred. Thereafter, the sol can be processed
into a membrane.

EXAMPLE 1.2

[0069] A glass beaker is charged with 300 g of DM water together with 50 g
of ethanol and 1.2 g of zirconium acetylacetonate. 280 g of
Al.sub.2O.sub.3 (MR 32 from Martinswerke) are then added a little at a
time with stirring. The suspension thus obtained is stirred for about 24
h. 150 g of zirconium sol are then added and the whole system is
thoroughly stirred. Thereafter, the sol can be processed into a membrane.

EXAMPLE 1.3

[0070] 70 g of tetraethoxysilane are hydrolyzed with 20 g of water and
peptized with 20 g of 25% nitric acid. This solution is stirred until
clear and after addition of 70 g of amorphous silica (Aerosil 90 from
Degussa) is stirred until the agglomerates have dissolved; the suspension
is subsequently used.

EXAMPLE 1.4

[0071] A glass beaker is charged with 300 g of DM water together with 50 g
of ethanol and 1.2 g of zirconium acetylacetonate. 450 g of
Al.sub.2O.sub.3 (ct 1200 from Alcoa, Ludwigshafen) are then added a
little at a time with stirring.

[0072] The suspension thus obtained is stirred for about 24 h. 150 g of
zirconium sol (from MEL) are then added and the whole system is
thoroughly stirred.

[0073] Thereafter, the sol can be processed into a membrane.

EXAMPLE 1.5

[0074] 100 g of silica sol (Levasil 200 from Bayer AG) were stirred with
180 g of M07 aluminum oxide from Sumitomo Chemical until the agglomerates
had dissolved. The suspension can then be used.

EXAMPLE 2.1

[0075] A glass fiber fabric woven from 11 tex yarns (S2-glass from AGY)
with 10 to 30 warp threads/cm and 10 to 30 weft threads/cm was treated at
450.degree. C. in the presence of atmospheric oxygen for 1 min to burn
off the size. The fabric was sprayed with a TEOS solution consisting of
1.5% of precondensed TEOS in ethanol and the solution was subsequently
dried at 50.degree. C. After drying, the fabric was treated once more at
450.degree. C. in the presence of atmospheric oxygen for 1 min. This
provided a silica-coated glass fiber fabric. (The same procedure can be
used when coating with ZrO.sub.2 (using zirconium acetylacetonate in
ethanol) and Al.sub.2O.sub.3 (using aluminum ethoxide in ethanol), both
from Merck.)

EXAMPLE 2.2

[0076] Example 2.1 was repeated using a fabric woven from a 5.5 tex yarn
(E-glass from AGY) with 5 to 30 warp threads/cm and 5 to 30 weft
threads/cm and a mixture of 1% TEOS and 0.5% of titanium
tetraisopropoxide (from Merck). This provided a distinctly thinner glass
fabric coated with an oxide mixture.

EXAMPLE 2.3

[0077] A fabric as from example 2.1 was desized by burning off for 1 min
in the presence of atmospheric oxygen and subsequent heat treatment at
300.degree. C. for 4 days. This provided a glass fiber fabric without
size.

EXAMPLE 2.4

[0078] A fabric as from example 2.2 was desized by burning off for 1 min
in the presence of atmospheric oxygen and subsequent heat treatment at
300.degree. C. for 4 days. This provided a glass fiber fabric without
size.

EXAMPLE 3.1

[0079] A suspension according to example 1.1 is knife coated onto a fabric
as per example 2.1 and dried in the course of 7 sec by blowing with hot
air at 450.degree. C. This provided a sheetlike separator which was
bendable down to a radius of 10 mm without damage. The Gurley number of
this separator, which had a pore size of about 100 nm, was 15.

EXAMPLE 3.2

[0080] A suspension according to example 1.2 is knife coated onto a fabric
as per example 2.3 and dried in the course of 7 sec by blowing with hot
air at 450.degree. C. This provided a sheetlike separator which was
bendable down to a radius of 10 mm without damage. The Gurley number of
this separator, which had a pore size of about 80 nm, was 19.

EXAMPLE 3.3

[0081] A suspension according to example 1.3 is knife coated onto a fabric
as per example 2.4 and dried in the course of 7 sec by blowing with hot
air at 450.degree. C. This provided a sheetlike separator which was
bendable down to a radius of 25 mm without damage. The Gurley number of
this separator, which had a pore size of about 50 nm, was 33.

EXAMPLE 3.4

[0082] A suspension according to example 1.4 is knife coated onto a fabric
as per example 2.3 and dried in the course of 7 sec by blowing with hot
air at 450.degree. C. This provided a sheetlike separator which was
bendable down to a radius of 25 mm without damage. The Gurley number of
this separator, which had a pore size of about 250 nm, was 5.

EXAMPLE 3.5

[0083] A suspension according to example 1.1 is knife coated onto a fabric
as per example 2.2 and dried in the course of 7 sec by blowing with hot
air at 450.degree. C. This provided a sheetlike separator which was
bendable down to a radius of 10 mm without damage. The Gurley number of
this separator, which had a pore size of about 100 nm, was 12.